Plate tectonics don't renew the whole surface, only the deep ocean part. The continents (including the continental shelf) are too light to be pulled down in the subduction zones. If Venus once had plate tectonics the "continents" (highlands) should be more heavily cratered than the "oceans" (basins). Whether this also implies that fossils and carbonates should be preferentially sought for in the highland areas is uncertain. The last life would have been found in the deepest parts of the basins, but these may have been "reprocessed" before plate tectonics stopped (here on Earth the ocean bottoms are completely "reprocessed" after ca 200 million years).

Incidentally there is evidence that Earth also had a "moist hothouse", not once but three times and that it saved rather than extinguishing life here. In the Late Proterozoic (600-800 million years ago) Earth suffered a series of extreme glaciations when all, or almost all, oceans froze over and continents were glaciated even in near-equatorial areas. Such a "snowball Earth" is climatically stable since the high albedo reflects most solar radiation back into space. However volcanism continued and since the oceans were ice-covered and the continents frozen no CO2 could be absorbed, but rather kept accumulating for maybe 20-30 million years. Temperatures slowly rose until the ice finally started melting, the albedo went down, the ice melted faster etc in a runaway process that converted Earth from Super-Antarctica to Super-Tropics in just a few thousand years. In this extreme hot-wet environment chemical weathering became intense and CO2 was rapidly drawn down and vast amounts of carbonates were deposited right on top of glacial deposits - a most unusual juxtaposition.

As for whether fossils could survive such extreme conditions, the answer is probably yes. Fossils can occasionally be recognizable in rocks that have been heated to similar temperatures on Earth. However here such heating is invariably linked to great depth and extreme pressures and also not continued for such a long period (500 mya), so it is difficult to make comparisons.tty

This is why one of the biggest goals in Venusian exploration is whether there is anything on Venus that can be called "continents" -- that is, patches of lightweight granitic rocks floating on top of the basalt plates, and therefore resistant to being pulled down into the deep by crustal-plate subduction. Earth's continents are thought to be made of lightweight silica-rich melt rock that was produced and separated when Earth's original basalt was mixed with large amounts of liquid water while it was being pulled down into the asthenosphere and remelted -- so the existence and size of any continents on Venus is yet another factor that seems to depend on the size of any initial liquid-water oceans it had early on. (The 2002 Decadal Survey is very clear on this point -- find any large amounts of granite on Venus anywhere, and you have strong evidence that it had substantial water oceans early on.)

Thus the strong interest in using the future landers to inspect the two types of Venusian terrain suspected of being possible continents; the tesserae (which were the target of one of the twin landers in Donald Esposito's "SAGE" concept at the last New Frontiers submission), and the huge "Ishtar Terra" in Venus' north polar region.

(By the way, the JPL technical report on SAGE -- http://trs-new.jpl.nasa.gov/dspace/bitstre...4/1/03-2520.pdf -- implies (pg. 18 and 21) that the only reason it was rejected as a finalist is simply that its launch window to Venus didn't fit in with the assigned period during which NF 2 was supposed to be launched. If that's the only grudge they had with it, I think it will definitely be a front-runner for the NF 3 selection, whenever that is finally made -- espcially since the Inner Planets Subgroup of the 2002 Decadal Survey ranked a Venus lander as more important than a sample-return mission to the moon's Aitken Basin, and the only reason the Survey as a whole gave the latter such a high overall rating is that it said the automatic rendezvous and docking technology that they thought an Aitken Basin mission would need would be useful practice for a Mars sample return. As things turned out, the Aitken Basin proposal that was a finalist -- "Moonrise" -- didn't use unmanned R&D at all.)

I've read quite a bit about the two supposed "Snowball Earth" periods in the Precambrian, and the wild pogo-sticking in global temperature that's thought to have occurred during them. There are several explanations proposed for them -- ranging from continental drift breaking up an initial giant supercontinent to create a bunch of smaller continents with a larger total amount of shallow coastlines that thus pulled more CO2 out of the air by carbonate weathering, to the idea that the first photosynthetic cyanobacteria may have brought on one of the two crises themselves by producing enough oxyen to destroy the methane greenhouse that had been keeping early Earth warm. (And the latter, in turn, may perhaps have been originally created by the earlier generation of methanogenic bacteria!) There are still a few holdouts on whether the Snowball Earth episodes occurred at all, but the evidence seems to be growing steadily.

And as for the survival of fossils -- or at least microfossils -- under the savage surface conditions of Venus, there's a rather encouraging new LPSC abstract ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1028.pdf ) on the apparent ease with which biochemical fossil evidence seems to survive even in giant-meteor impact melts on Earth. But if you think a fossil hunt on Mars will be difficult, consider one on Venus...

Thank you Bruce for reporting on Mr. Grinspoon's ideas. I think that he is one of the more original and innovative members of the planetary science community. I especially like David Grinspoon's proposal to include a manned mission to Venus, as part of the VSE. As I recall, his plan calls for a crew to orbit Venus in a CEV and use that vantage point to control a series of unmanned probes on the planet itself.

I'm not really a fan of either of his ideas. Carl Sagan suggested a long time ago that (Earth-born) microbes might be bred to survive in the Venusian clouds. Um, he probably says "Cytherean clouds". But that was in the early 1960s, before we knew better.

I'd be surprised if any molecule with more than a couple carbon atoms can survive in the Venusian atmosphere. Certainly there is no obvious sign of organics in the IR fourier-spectrometer measurments (Venera-15), while the Earth has clear CH-band structures. Keep in mind, the atmosphere of Venus is very dynamic-- convenction cells, high speed winds at the cloud level, etc. Those hapless microbes will alternate between being carried up into the zone of solar radiation, with no ozone or magnetosphere to protect them from every kind of ionizing radiation. Then down they go into the deep atmosphere to be pressure-cooked in suphuric acid. It's the kind of theory you propose to get your name spashed in the papers.

Then there is the manned orbiter around Venus. Kinda like ISS...except more expensive and dangerous. OK, now I said it, ..."ISS"

NASA is spending 4 times as much money as ESA, RKA and China combined. For $17 billion a year, just think about the kinds of science we could be doing all over the solar system. Rovers on IO, sample return missions to the planets and satellites, submarines under Europa. But here we with NASA wringing its hands, cancelling missions. Now why is that?

"After having some business dealings with men, I am occasionally chagrined, and feel as if I had done some wrong, and it is hard to forget the ugly circumstance. I see that such intercourse long continued would make one thoroughly prosaic, hard, and coarse. But the longest intercourse with Nature, though in her rudest moods, does not thus harden and make coarse. A hard, sensible man whom we liken to a rock is indeed much harder than a rock. From hard, coarse, insensible men with whom I have no sympathy, I go to commune with the rocks, whose hearts are comparatively soft."

So what exactly was the purpose of those Police boxes in the UK? Kind of an exclusive telephone booth for cops?

Yes - from the days before portable radios (they were designed in the 1920s). UK Police also carried whistles with a very distinctive sound, to call for aid (I always wondered what stopped the guys in the black hats getting hold of whistles in order to confuse matters, sorta like a 'whistle gap'!). Officer Dribble, er Dibble, in the classic Boss Cat cartoon show had exactly the same thing, but in his case it was just a phone in Top Cat's alley.

After clawing my way through my CD-ROM library of recorded Web documents on Venus, I find the following on Grinspoon's suggestion that Habitable Venus may have lasted a very long time. (There certainly must be more than these.)

I found the presentation on the Japanese Venus Climate Orbiter and the Flagship mission analysis particularly interesting. The former is an imaging mission that will use a near equatorial orbit to achieve semi-synchronous coverage of the atmospheric rotation of the atmosphere.

The flagship report details the analysis for a possible 2020's (or so hoped) mission. As reported in the press, the proposal would include an orbiter (high resolution radar seems to be the key instrument), two landers with atmospheric composition capabilities and balloon ascent following sample acquisition, and two mid-altitude balloon platforms. Total cost is ~$3B (presumably in today's dollars).

Some thoughts on the flagship proposal:

The proposal seems to knock the legs out from under the New Frontiers VISE proposal, or any similar New Frontiers proposal. (This involved two short lived landers without balloon ascent.) This scale of mission receives the lowest science score. Yet for the price of two of VEXAG's short-lived landers (with balloon ascent), 4-6 VIS landers could be implemented.

The orbiter is actually fairly cheap, almost Discovery class, and would probably fit within that budget with some modest international participation. There's no claim in the presentation that there is much advantage to flying the orbiter with the other Flagship elements.

All medium altitude balloon options seem to cost ~$2B, as do the short-lived landers with balloon ascent.

VEXAG appears to be going to the full monty with a Flagship proposal. Yet, at best, Venus would be third in line after an outer planets flagship and a Mars sample return. Never hurts to make your request. But I would have two requests outstanding -- a high priority Discovery and a New Frontiers mission (fly something in the next decade) and a follow on flagship that might fly in the 2020s, and more likely (in my opinion) in the 2030s.

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